linux-kernel - Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start

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Date:	Sat, 23 Aug 2014 19:22:40 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Julien Tinnes <jln@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack
 check

On Sat, Aug 23, 2014 at 04:43:27PM +0200, Oleg Nesterov wrote:
> The ->start_stack check in do_shmat() looks ugly and simply wrong.
> 
> 1. ->start_stack is only valid right after exec(), the application
>    can switch to another stack and even unmap this area.
> 
> 2. The reason for this check is not clear at all. The application
>    should know what it does. And why 4 pages? And why in fact it
>    requires 5 pages?
> 
> 3. This wrongly assumes that the stack can only grown down.
> 
> Personally I think we should simply kill this check, but I did not
> dare to do this. So the patch only fixes the 1st problem (mostly to
> avoid the usage of mm->start_stack) and ignores VM_GROWSUP.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@...il.com>

I don't understand this check either, the comment above it says nothing
but only commits what code is doing not explaining why.
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